In order to effectively use electric power, a power supply system configured by combining a commercial power supply with a storage battery has begun to come into use. That is, in the power supply system, according to temporal variations of a load, the power supplied from the commercial power supply is temporally averaged in such a manner that, when the load is large, the discharge power from the storage battery is supplied to the load in addition to the power supplied to the load from the commercial power supply, and that, when the load is small, the storage battery is charged by supplying power from the commercial power supply to the storage battery. Further, a solar power generation system and a fuel cell system, which have been developed in recent years, are also combined with the power supply system.